DC to AC converters such as ring generators are well known devices in common use today for providing the ringing signals used to ring a standard telephone. Telephone service providers employ them to generate the standard ringing tone sent to the telephone(s) installed at the customer's premises. In addition, manufacturers of telephone PBX equipment employ ring generators to provide the ringing signals used to ring the extension telephone sets connected to the PBX equipment.
Typical ring generators are designed to provide sinusoidal or substantially sinusoidal waveforms. Smaller telephone systems, e.g., small loop carrier systems, PBX or key systems, generally make use of electronic ringing techniques as opposed to designs employing large iron core transformers. These smaller systems are typically required to be able to ring a large percentage of the lines simultaneously.
Further, more and more optical fiber is being introduced into the telephone loop plant which the associated increased need for small, reasonably efficient ring generators having well controlled sinusoidal output waveforms. Systems such as fiber to the curb (FTTC) typically serve four residences and may provide up to 12 telephone channels. Fiber to the home (FTTH) systems serve only one residence and typically provide less than three telephone channels. These and other related telephone systems are required to provide a source of ringing which is capable of ringing up to three lines simultaneously. In addition, each line may have as many as five ringers associated with it. The output of the ring generator device must be sufficient to drive each of the ringers on all the supported telephone lines with reasonable efficiency and must maintain a sinusoidal wave shape.
In addition, the ring generator device must be able to be powered from power sources of varying input voltage, such as the commonly found voltages of 5, 12, 24 or 48 Vdc. The power consumption of such a device should be limited so as not to draw excess power and deprive other power supplies in the system of sufficient power to operate satisfactorily. The ring generator device should also be capable of being subjected to overcurrent, i.e., low resistance fault, and short circuit conditions without being sustaining permanent damage. If the device is subjected to an overload due to an excessive number of ringers being rung simultaneously, it must continue to operate and output a sinusoidal waveform, even if it must reduce the output voltage to a point that is not sufficient to ring the excessive ringer load.